On Saturday, the retirement countdown clocks offered by Microsoft and others flipped from 500 to 499 days, or a shade under 17 months.

Microsoft provides a countdown gadget for Windows XP's support demise. Ironically, the gadget runs only on Windows 7, the 2009 OS that most customers have adopted after departing XP.

Camwood, a U.K.-based company that specializes in helping businesses migrate their machines to newer operating systems and software, has posted a similar clock on its website. Like Microsoft's gadget, Camwood's also showed 499 days remaining on Saturday.

When Microsoft pulls XP's plug, it will have maintained the operating system for 12 years and five months, or about two-and-a-half years longer than its usual practice. That's also a record, replacing the previous Methuselah, Windows NT, which received 11 years and 5 months of support.

XP's long life was caused in large part by the debacle that was Windows Vista, an oft-delayed operating system that was ultimately rejected by most XP users for being buggy, sluggish or lacking in driver support. Instead, those customers waited for the next iteration, Windows 7, which has been as much a success as Vista was a failure.

According to Web metrics company Net Applications, Windows XP powered 40.7% of the world's desktop and notebook personal computers that went online last month. Windows 7, which passed its ancestor only in August, held a usage share of 44.7% in October.

Meanwhile, Windows Vista, which peaked at 19.1% in October 2009, the same month Windows 7 debuted, now accounts for just 5.8% of all systems.

Microsoft has remained adamant that XP will exit support in April 2014, and has urged customers to upgrade as soon as possible. But countdown clocks notwithstanding, analysts have predicted that XP will be used by millions well after that deadline.

Last month, for instance, Gartner analyst Michael Silver said "there's a good chance" that between 10% and 15% of enterprise PCs will be running XP after April 2014.

Computerworld's forecasts have been overly optimistic about XP's decline. In mid-2011, Computerworld predicted that Windows XP would account for 38% in the third quarter of 2012, three percentage points lower than the eventual number.

Current estimates based on Net Applications' data indicate that come April 2014, Windows XP will be running between 27% and 29% of the world's computers.

"The end of XP support is a potential time bomb," Camwood said last week. "And the clock is ticking."

Windows XP users who want to create a countdown clock on their desktops can install one of several free utilities -- including TimeLeft from Canadian developer NesterSoft -- then set the countdown target at 10 a.m. PT on April 8, 2014. (Microsoft shoots for a 10 a.m. PT release for each month's security updates.)